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Speaker Asylum General speaker questions for audio and home theater. |
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Speaker Asylum General speaker questions for audio and home theater. |
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In Reply to: RE: Rear Firing Ports & Placement posted by Duke on November 22, 2007 at 00:21:27:
Dear Duke,
There are really two issues here,
A.) The placement of speakers close to room boundaries
B.) The subsequent use of the boundaries for bass reinforcement
All the early research I have seen from Western Electric, RCA, Klangfilm and others plus later work done by Philips (the early corner speakers), Klipsch, Voigt & Chave, Allison and even Beveridge, point to the fact that sound waves being propagated from a corner or close to the room boundaries have a far more similar behaviour in different rooms than if the speakers were placed away from the walls in the same rooms, admittedly this work was done in the early days of cinema, but there are no reasons why it should not apply to someone's living room, the laws of acoustics haven’t changed since then to my knowledge, and in my experience this still applies.
Just by way of one example, I took part in an experiment that was sponsored by Bruel & Kjaer, at the main Danish engineering university at Lundtofte in 1981, where 4 pairs of speakers were placed in different rooms in various positions, then measured and listened to, we used a pair of Quad ESL63s, B&W 801, a pair of JBLs the model of which escapes me and a pair of SNELL Type A/IIs.
By a not insignificant margin, the general consensus was that the SNELL A's where least affected by the different rooms when close to the walls and was, by a lesser margin, preferred overall, both on the basis of the measured performance and by the listeners, followed by the Quads, the 801s and the JBLs.
Just for the record, the A/II's are floor loading the woofer, a fact which was considered to be a major reason for their good general in room performance, their excellent dispersion was another.
Our experience clearly demonstrates that provided the speaker has a good, wide and even dispersion window, meaning it projects an almost even hemispherical waveform into the room (something which modern narrow baffle speakers do not, which easily explains why they do not work close to room boundaries) then near wall and corner placement even with a sealed box markedly improves the room to room consistency and general behaviour, I see several reasons for this,
1.) When a sound source (read, a speaker) is placed close to the room boundaries the frequency content of the signal is reflected in a more predictable and consistent manner from room to room, which greatly reduces room interference. When the speaker is close to the side walls it is not possible for the ear to separate the reflected and direct sound from each other, and results in a great reduction in room variability.
2.) What the near side wall placement also does is increase the in room efficiency of the speaker, this is because the off axis energy is now added to the overall sound output and because the frequency energy from a speaker which has good wide dispersion is almost the same at say 30 and 60 degrees, this energy does not alter or skew the overall frequency response, a fact which also explains why speakers with narrow baffles and deep cabinets do not display this ability even if the port is moved to the back of the cabinet, the dispersion is simply too uneven off axis.
3.) Having the port in the back, and dimensioning it to take into consideration the additional loading available from the rear wall/corner/floor (the floor by placing it close to the bottom of the speaker cabinet) not just improves the low frequency extension by up to 18dB, it also allows an unprecedented degree of adjustability of in room bass response. The speaker pressurizes the room in a completely different way in the corner position using this technique when compared to the way a woofer in even a much larger cabinet with either a front facing port or a sealed cabinet
All in all there are many good reasons why near room boundary placement enhances the performance of a speaker designed to be used in this position, so whilst I can see some reasons for why the audio industry standard measurement require free field on axis measurements I believe these were chosen out of convenience and for the sake of having an easily repeatable method, but I think it is overall a no less misleading way of “proving†speaker performance because we all know (and so did the engineers who originally decided on the anechoic measurement technique) that all speakers end up in a room with four walls and floor and a ceiling, so why did they not establish a technique which better reflects that?
My view of this is that as with many other bench mark tests of audio performance that what was sought was not the best way of getting a real measure of performance, but the easiest way of measuring something that could be used as a marketing tool, and off course because the overwhelming majority of speaker manufacturers use this measure to design their speakers they have no interest in rocking the boat or designing speakers which do not conform to the standard tests, because it would mean comparing poorly with their direct competitors, but does the fact that most speaker manufacturers uses the same method to design and measure their speakers mean that this method is the best available or produces the best end results??
If it not the job of the magazines to question the practices of the industry they write about, rather than pander to its need to sell its products, what is their job??
In my view it is the main job of the press to criticize and point out poor practice and that applies not just to audio (may I say here that many brave and committed journalists die every year exposing deplorable practices in many parts of the world and their sacrifices should be recognised and are not remotely comparable to the minor transgressions of their audio brethren), but magazines like Stereophile and I should say for the record that Stereophile are by no means the only or the main culprit here, should be pointing out the weaknesses in the methods used by the industry they are there to be policing on behalf of their readers in the hope/expectation that it would encourage the industry to improve its processes and as a result its product over the longer terms.
I just don’t see this happening, not in the 30 odd years I have worked on my "project", the magazines are almost greater slaves to convention than most manufacturers, they jump every new bandwagon almost before any manufacturers have had time to embrace it, thus forcing most larger manufacturers to embrace technology which in many cases turns out to dud wasting vast amounts o their readers money in the process, not really a commendable record is it?
Wolves should not be game keepers and perhaps this lack of ability/willingness to criticize explains the slow death of most of the printed magazines as readers turn their back to them and start looking elsewhere?
Anyway, enough of this hope it put a little more information on the bone so to speak!
Sincerely,
Peter Qvortrup
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Follow Ups
- Room Placement & Rear Ported Speakers. - Peter Qvortrup 03:32:47 12/16/07 (0)
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